This invention relates to a method for producing an integral titanium alloy article having at least two regions, each region having a distinct microstructure and mechanical properties.
Metal fatigue is one of the major causes for failure of aerospace vehicles and gas turbine engines. The fatigue damage of all technical alloys is divided into two stages: fatigue crack initiation and fatigue crack propagation. In most alloys, the best resistance to fatigue crack initiation is obtained when the material has a very fine microstructure. The best resistance to the propagation stage is associated with a relatively coarse microstructure.
In the alpha+beta titanium alloys, the most crack initiation resistant microstructure is a fine equiaxed alpha structure, while the most crack propagation resistant microstructure is associated with large groups or colonies of high aspect ratio alpha phase plates.
Relatively thick alpha+beta alloy castings generally have a transformed beta microstructure, i.e., an alpha plate microstructure. Mill annealed wrought alpha+beta titanium alloys generally have a microstructure which is a relatively coarse mixture of low aspect ratio alpha separated by a small amount of intergranuluar beta. Articles prepared by consolidation of prealloyed alpha+beta titanium alloy powder generally have a microstructure which is a mixture of high and low aspect ratio coarse alpha plates separated by a continuous beta phase.
The microstructure of wrought, cast and powder metallurgy articles can be altered or refined by a variety of methods, including forging, beta-solution heat treatment, annealing, hydrogenation/dehydrogenation, and the like. With forging, it is possible to locally refine microstructure, so that an article will have differing microstructures in different portions of the article through control of the amount of forging work. With other microstructures refining methods, it is generally either not possible or not feasible to refine the microstructure of an article is selected locations.
Vogt et al, U.S. Pat. No. 4,680,063 disclose that the microstructure of a forged titanium alloy article can be altered by a process comprising beta-solution heat treatment, hydrogenation, cooling, dehydrogenation and cooling. Smickley et al, U.S. Pat. No. 4,505,764 disclose that the microstructure of cast titanium alloy articles can be refined by a method comprising hydrogenation, then dehydrogenation. Levin et al, U.S. Pat. No. 4,655,855 disclose that the microstructure of prealloyed titanium alloy powder compacts can be refined by a method comprising beta-solution heat treatment, cooling to room temperature, hydrogenating and dehydrogenating. Yolton et al, U.S. Pat. No. 4,219,357 disclose a method for producing powder compacts having a fine grain size which comprises hydriding a titanium alloy powder, hot compacting the hydrided powder in a suitable mold, dehydriding the resulting article, reheating the dehydrided article to remove voids formed during dehydriding.
While the methods briefly described above represent valuable contributions to the art, most have the drawback that the microstructure of the resulting article is uniform. Smickley et al disclose that the surface of castings can be modified by limiting the hydrogen partial pressure or controlling the hydrogenation time at a given pressure to limit hydrogen addition to the surface regions. This method has the drawback that it would be difficult, if not impossible, to modify the microstructure of selected and isolated surface regions. What is desired is a method for producing a titanium allooy article having at least two microstructures. In particular, what is desired is a method for producing a titanium alloy article having one or more regions which are resistant to crack initiation and one or more regions which are resistant to crack propagation.
It is, therefore, an object of the present invention to provide a method for producing titanium alloy articles having high fatigue and fracture resistance.
Other objects of the invention will become apparent to those skilled in the art from a reading of the following description of the invention.